This NIH BRAIN Initiative R21 will initiate development of a completely Implantable Brain Microelectromechanical Magnetic Sensing and Stimulation (MEMS-MAGSS) technology. Significance: We seek to offer proof-of-concept testing and development of a novel class of MEMS-MAGSS technology, to address the NIH BRAIN Initiative: New Concepts and Early-Stage Research for Large-Scale Recording and Modulation in the Nervous System (R21). The current state of the art for large-scale recording of neuronal activity does not have cellular resolution for sensing and stimulation. The current state of the art for highly sensitive magnetic sensing cannot be performed at safe temperatures for biological implantation, and requires expensive shielded rooms incompatible with human long-term use. Innovation: Magnetic fields can now be sensed at amplitudes and spatial density never before possible using several new microelectromechanical electrical systems (MEMS) technologies that we have pioneered. We are in a unique position to create a next generation of magnetic sensing and stimulation devices capable of meeting the high-density cellular level mandate of the NIH BRAIN Initiative. We have brought together a unique team of exceptional investigators in electrical engineering, physics, neurophysiology, neurosurgery, and materials science with the requisite skills to collaborate in a highly integrated transdisciplinary fashion capable of meeting the proof-of-concept milestones of this project within 2-years. Approach: We will approach this project by selecting from among 2 magnetic sensing technologies already in the prototype stage. We can incorporate on-chip adaptive magnetic noise cancellation for ambient magnetic fields. Using prototype MEMS based magnetic stimulation designs, we will develop the ability to integrate simultaneous magnetic sensing and stimulation. We will perform proof of concept experiments disambiguating the magnetic signatures from single cell firing in cortical brain slices, and establish both forward and inverse solutions of these same neurons. Impact: This project would produce a 'first-of-kind' technology capable of 1) cellular resolution detection of spiking activity in neurons, 2) cellular level modulation of neuronal firing, 3) adaptve noise cancellation enabling use outside of magnetically shielded environments, 4) room-temperature operation enabling packaging for long-term implantation within with biological tissue for animal or human use, and 5) a clear translational pathway for long-term human implantation across a person's life-span.